Blending cardiac rhythm detection processes

ABSTRACT

Systems and methods are described for classifying a cardiac rhythm. A cardiac rhythm is classified using a classification process that includes a plurality of cardiac rhythm discriminators. Each rhythm discriminator provides an independent classification of the cardiac rhythm. The classification process is modified if the modification is likely to produce enhanced classification results. The rhythm is reclassified using the modified classification process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.12/895,151, filed on Sep. 30, 2010, which is a Continuation of U.S.application Ser. No. 11/089,185, now issued as U.S. Pat. No. 7,818,056,filed on Mar. 24, 2005, the benefit of priority of which are claimedherein, and which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to classifying, with an implantable medicaldevice, cardiac rhythms produced by the heart.

BACKGROUND OF THE INVENTION

Proper cardiac function relies on the synchronized contractions of theheart at regular intervals. When normal cardiac rhythm is initiated atthe sinoatrial node, the heart is said to be in sinus rhythm. However,due to electrophysiologic disturbances caused by a disease process orfrom an electrical disturbance, the heart may experience irregularitiesin its coordinated contraction. In this situation, the heart is denotedto be arrhythmic. The resulting cardiac arrhythmia impairs cardiacefficiency and can be a potential life threatening event.

Cardiac arrhythmias occurring in the atria of the heart, for example,are called supra-ventricular tachyarrhythmias (SVTs). SVTs take manyforms, including atrial fibrillation and atrial flutter. Both conditionsare characterized by rapid, contractions of the atria. Cardiacarrhythmias occurring in the ventricular region of the heart, by way offurther example, are called ventricular tachyarrhythmias. Ventriculartachyarrhythmias (VTs), are conditions denoted by a rapid heart beat,150 to 250 beats per minute, originating from a location within theventricular myocardium.

Ventricular tachyarrhythmia can quickly degenerate into ventricularfibrillation (VF). Ventricular fibrillation is a condition denoted byextremely rapid, non synchronous contractions of the ventricles. Thiscondition is fatal unless the heart is returned to sinus rhythm within afew minutes.

Implantable cardioverter/defibrillators (ICDs) have been used as aneffective treatment for patients with serious tachyarrhythmias. ICDs areable to recognize and treat tachyarrhythmias with a variety of tieredtherapies. These tiered therapies range from providing anti-tachycardiapacing pulses or cardioversion energy for treating tachyarrhythmias tohigh energy shocks for treating ventricular fibrillation. To effectivelydeliver these treatments, the ICD must first identify the type oftachyarrhythmia that is occurring, after which appropriate therapy maybe provided to the heart.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading thepresent specification, there is a need in the art for reliably andaccurately recognize types of cardiac rhythms produced by the heart. Thepresent invention fulfills these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to a method and system for classifyingcardiac rhythms using an implantable device. An embodiment of theinvention involves a method for classifying a cardiac rhythm. Thecardiac rhythm is classified using a classification process thatincludes a plurality of cardiac rhythm discriminators. Each rhythmdiscriminator provides an independent classification of the cardiacrhythm. The method determines if modifying the classification process islikely to enhance classification. If so, the classification process ismodified and the rhythm is reclassified using the modifiedclassification process.

Another embodiment of the invention is directed to device forclassifying cardiac rhythms. The system includes a sensing circuitconfigured to sense cardiac signals associated with a cardiac rhythm. Aplurality of independent rhythm discriminators are configured to providean independent classification of the cardiac rhythm. The system furtherincludes a classification processor coupled to the independent rhythmdiscriminators. The classification processor configured to implement aclassification process to classify the cardiac rhythm, theclassification process based on results of the independent rhythmdiscriminators. The classification processor is further configured todetermine if modification of the classification process is likely toenhance rhythm classification and to modify the classification processif the rhythm classification is likely to be enhanced. Theclassification processor is configured to reclassify the cardiac rhythmusing the modified classification process.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method of classifying a cardiacrhythm according to embodiments of the invention;

FIG. 2 is a partial view of a cardiac rhythm management (CRM) devicethat may be used to implement arrhythmia classification and therapy inaccordance with embodiments of the invention;

FIG. 3 is a block diagram of a cardiac rhythm management (CRM) device300 suitable for implementing arrhythmia classification and therapydelivery in accordance with embodiments of the invention;

FIG. 4 is a flow diagram illustrating a method of classifying a cardiacrhythm in accordance with embodiments of the invention;

FIG. 5A is a flowchart illustrating classification of a cardiac rhythmusing multiple independent discriminators in accordance with embodimentsof the invention;

FIGS. 5B and 5C are graphs illustrating the process of rate and shockchannel alignment used in morphology-based discriminator in accordancewith embodiments of the invention;

FIG. 6A is a graph illustrating regions of the A-V rate plane that maybe utilized for classification of the cardiac rhythm in accordance withembodiments of the invention;

FIGS. 6B, 7, 8A, 9, 10, 11, and 12 are flowcharts illustrating a rhythmclassification process utilizing at least one morphology-based rhythmdiscriminator and at least one interval-based rhythm discriminator inaccordance with embodiments of the invention; and

FIG. 8B is a graph illustrating modification of a correlationcoefficient used for morphology-based rhythm discrimination inaccordance with embodiments of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail hereinbelow. It is to beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

Cardiac therapy devices such as implantable cardiac defibrillatorsand/or cardioverters recognize various cardiac rhythms and providetreatment to convert, interrupt or mitigate dangerous rhythms. A tieredapproach to therapy may be implemented, wherein some rhythms are treatedwith anti-tachycardia pacing (ATP), other rhythms are treated with highenergy defibrillation shocks, and some arrhythmias are left untreated.

Appropriate treatment for cardiac arrhythmias may be selected based inpart on the origin of the arrhythmia. For example, a fast, disorganizedrhythm arising in a ventricle may be recognizable by an implantablecardiac device as ventricular fibrillation, a dangerous condition thatrequires immediate treatment. In this scenario, one or moredefibrillation shocks may be applied to the heart to terminate thefibrillation and restore the heart to sinus rhythm.

If a ventricular rhythm is rapid but organized, and is associated with arelatively stable morphology, then a first therapy such as,anti-tachycardia pacing (ATP), may be delivered to the ventricles. Ifthe ATP therapy is unsuccessful, then a different therapy may bedelivered, such as one or more defibrillation shocks, may be delivered.

A fast rhythm arising in the atria, such as atrial fibrillation oratrial flutter, may be recognized by the device and may be treated withpacing or cardioversion shocks or may be left untreated. Fast atrialrhythms are generally not immediately life threatening and may notrequire treatment.

A number of cardiac rhythm discrimination procedures, denoted herein asrhythm discriminators, may be implemented in an implantable cardiacdevice or other therapy system to recognize the type of cardiacarrhythmia experienced by the patient. The embodiments of the inventionpresented herein are directed to systems and methods involving the useof a plurality of independent rhythm discriminators to recognize orclassify various types of cardiac arrhythmia.

Classifying a cardiac rhythm using more than one independent rhythmdiscriminator may be desirable to increase the accuracy and/orsensitivity of the rhythm classification. However, the use of multiplediscriminators adds complexity to the system that requires additionalprocessing. For example, classification of the cardiac rhythm usingmultiple independent discriminators may produce conflictingclassification results. Modification of the classification process, suchas by using additional information acquired by other device processes,by modifying parameters of the initially used discriminators, byemploying additional discriminators, or by other procedures, may berequired or desired to resolve conflicts between classification results.

Further, enhancement of the rhythm classification process may bedesirable even though the independent discriminators do not produceconflicting classifications. The device may be capable of recognizing ifmodification of the classification process is likely to enhanceclassification and modifying the classification process. For example,enhancement of the classification process may be desirable if the one ormore discriminators are produce an indeterminate or borderlineclassification result. In this scenario, the device may enhanceclassification by modifying the classification process. Theclassification process may be modified, for example, by using additionalinformation acquired by other device processes, by modifying parametersof the initially used discriminators, by employing additionaldiscriminators, or by other procedures.

The flowchart of FIG. 1 illustrates a method of classifying a cardiacrhythm according to embodiments of the invention. Initially, the cardiacrhythm is classified 110 using a plurality of independent rhythmdiscriminators. The rhythm discriminators may classify a cardiac rhythmbased on various characteristics of the cardiac rhythm including, forexample, the rate of the cardiac rhythm, intervals between successiveatrial and/or ventricular beats, and the morphology of sensed cardiacsignals associated with cardiac beats of the rhythm. The rhythmdiscriminators typically utilize one or more parameters that may beprogrammable by the physician and/or alterable by the device.

After the cardiac rhythm has been classified 110 by the initial processusing the independent rhythm discriminators, the device determines 112if enhancement of the rhythm classification is required or desired. Ifrhythm classification enhancement is not desired, then the cardiacrhythm is classified 113 based on the initial classification process.For example, in one scenario, the device may implement enhanced rhythmclassification if two of the cardiac rhythm discriminators used in theinitial rhythm classification process return conflictingclassifications. In another scenario, the device may implement enhancedrhythm classification if one or more of the rhythm discriminators wasunable to classify the cardiac rhythm and returned an indeterminateresult. In yet another scenario, the device may implement enhancedrhythm classification if the initial rhythm discriminators returnedborderline classification results and additional and/or modifiedprocessing may be desired or required to confirm the initialclassification.

If enhanced rhythm classification is implemented, then the process forperforming the rhythm classification may be modified 114 and the rhythmis reclassified 115 using the modified classification process. Invarious embodiments described in greater detail below, modification ofthe classification processes may involve modifying one or moreparameters associated with a particular rhythm discriminator, acquiringadditional information acquired by other functions of the cardiac deviceto aid in the classification processes, initiating additional rhythmdiscriminators to enhance the rhythm classification and/or by otherprocedures.

FIG. 2 is a partial view of a cardiac rhythm management (CRM) devicethat may be used to implement rhythm classification and arrhythmiatherapy in accordance with embodiments of the invention. Methods of theinvention may be implemented in a variety of implantable orpatient-external cardiac therapeutic and/or diagnostic devicesincluding, for example, pacemakers, defibrillators, cardioverters,bi-ventricular pacemakers, and/or cardiac resynchronization devices,among others. The CRM device illustrated in FIG. 2 includes animplantable housing 200 containing circuitry electrically coupled to anintracardiac lead system 202. Portions of the implantable housing may beconfigured as a can electrode 209. The housing 200 and the intracardiaclead system 202 is implanted in a human body with portions of theintracardiac lead system 202 inserted into a heart 201. The intracardiaclead system 202 is used to detect electric cardiac signals produced bythe heart 201 and to provide electrical energy to the heart 201 underpredetermined conditions to treat cardiac arrhythmias.

The intracardiac lead system 202 includes one or more electrodes usedfor pacing, sensing, and/or defibrillation. In the particular embodimentshown in FIG. 2, the intracardiac lead system 202 includes a rightventricular lead system 204, a right atrial lead system 205, and a leftventricular lead system 206. In one embodiment, the right ventricularlead system 204 is configured as an integrated bipolar pace/shock lead.

The right ventricular lead system 204 includes an SVC-coil 216, anRV-coil 214, and an RV-tip electrode 212. The RV-coil 214, which mayalternatively be configured as a separate defibrillation coil and anRV-ring electrode 211, is spaced apart from the RV-tip electrode 212,which is a pacing electrode for the right ventricle.

The right atrial lead system 205 includes a RA-tip electrode 256 and anRA-ring electrode 254. The RA-tip 256 and RA-ring 254 electrodes mayprovide pacing pulses to the right atrium of the heart and may also beused to detect cardiac signals from the right atrium. In oneconfiguration, the right atrial lead system 205 is configured as aJ-lead.

In the configuration of FIG. 2, portions of the intracardiac lead system202 are shown positioned within the heart 201, with the rightventricular lead system 204 extending through the right atrium and intothe right ventricle. Typical locations for placement of the RV-tipelectrode 212 are at the right ventricular (RV) apex or the RV outflowtract.

In particular, the RV-tip electrode 212 and RV-coil electrode 214 arepositioned at appropriate locations within the right ventricle. TheSVC-coil 216 is positioned at an appropriate location within a majorvein leading to the right atrium chamber of the heart 201. The RV-coil214 and SVC-coil 216 depicted in FIG. 2 are defibrillation electrodes.

The left ventricular lead system 206 is advanced through the superiorvena cava (SVC), the right atrium 220, the ostium of the coronary sinus,and the coronary sinus 250. The left ventricular lead system 206 isguided through the coronary sinus 250 to a coronary vein of the leftventricle. This vein is used as an access pathway for leads to reach thesurfaces of the left atrium and the left ventricle which are notdirectly accessible from the right side of the heart. Lead placement forthe left ventricular lead system may be achieved via subclavian veinaccess and a preformed guiding catheter for insertion of the leftventricular (LV) electrodes 213 and 217 adjacent the left ventricle. Inone configuration, the left ventricular lead system 206 is implementedas a single-pass lead.

An LV distal electrode 213, and an LV proximal electrode 217 may bepositioned adjacent to the left ventricle. The LV proximal electrode 217is spaced apart from the LV distal electrode, 213 which is a pacingelectrode for the left ventricle. The LV distal 213 and LV proximal 217electrodes may also be used for sensing the left ventricle.

The lead configurations illustrated in FIG. 2 represent one illustrativeexample. Additional lead/electrode configurations may include additionaland/or alternative intracardiac electrodes and/or epicardial electrodes.For example, in one configuration, an extracardiac lead may be used toposition epicardial electrodes adjacent the left atrium for deliveringelectrical stimulation to the left atrium and/or sensing electricalactivity of the left atrium.

Referring now to FIG. 3, there is shown a block diagram of a cardiacrhythm management (CRM) device 300 suitable for implementing arrhythmiaclassification in accordance with embodiments of the invention. FIG. 3shows a CRM device 300 divided into functional blocks. It is understoodby those skilled in the art that there exist many possibleconfigurations in which these functional blocks can be arranged. Theexample depicted in FIG. 3 is one possible functional arrangement.Various functions of the CRM device 300 may be accomplished by hardware,software, or a combination of hardware and software.

The CRM device 300 includes components for sensing cardiac signals froma heart and delivering therapy, e.g., pacing pulses or defibrillationshocks, to the heart. The circuitry of the CRM device 300 may be encasedand hermetically sealed in a housing 301 suitable for implanting in ahuman body. Power to the circuitry is supplied by an electrochemicalbattery power supply 380 that is enclosed within the housing 301. Aconnector block with lead terminals (not shown) is additionally attachedto housing 301 to allow for the physical and electrical attachment ofthe intracardiac lead system conductors to the encased circuitry of theCRM device 300.

In one embodiment, the CRM device 300 comprises programmablemicroprocessor-based circuitry, including control circuitry 320, amemory circuit 370, sensing circuitry 331, 332, 335, 336, and apacing/defibrillation pulse generator 341. Components of the CRM device300 cooperatively perform operations involving arrhythmia classificationaccording to the approaches of the present invention. The controlcircuitry 320 is responsible for arrhythmia detection, classification,and therapy control. The control circuitry 320 may encompass variousfunctional components, for example, independent rhythm discriminators361, 362, an arrhythmia classification processor 321 and a therapycontrol unit 322.

The memory circuit 370 may store program instructions used to implementthe functions of the CRM device 300 as well as data acquired by the CRMdevice 300. For example, the memory circuit 370 may store historicalrecords of sensed cardiac signals; including arrhythmic episodes, and/orinformation about therapy delivered to the patient. The memory circuit370 may also store morphology templates representative of cardiac beatsassociated with various types of cardiac rhythms.

The historical data stored in the memory 370 may be used for variouspurposes, including diagnosis of patient diseases or disorders. Analysisof the historical data may be used to adjust the operations of the CRMdevice 300. Data stored in the memory 370 may be transmitted to anexternal programmer unit 390 or other computing device, such as anadvanced patient management system as needed or desired.

Telemetry circuitry 360 allows the CRM device 300 to communicate with anexternal programmer unit 390 and/or other remote devices. In oneembodiment, the telemetry circuitry 360 and the external programmer unit390 use a wire loop antenna and a radio frequency telemetric link toreceive and transmit signals. In this manner, programming commands anddata may be transferred between the CRM device 300 and the externalprogrammer 390 after implant.

The CRM device 300 may function as a pacemaker and/or a defibrillator.As a pacemaker, the CRM device 300 delivers a series of electricalstimulations to the heart to regulate heart rhythm. Therapy controlcircuitry 322 controls the delivery of pacing pulses to treat variousarrhythmic conditions of the heart, for example. In various embodiments,the CRM device 300 may deliver pacing pulses to one or more of the rightatrium, left atrium, right ventricle and the left ventricle. The heartmay be paced to treat bradycardia, or to synchronize and/or coordinatecontractions of the right and left ventricles.

For example, right ventricular pacing may be implemented using unipolaror bipolar configurations. Unipolar RV pacing involves, for example,pacing pulses delivered between the RV-tip 212 to can 209 electrodes.Bipolar pacing involves, for example, delivery of pacing pulses betweenthe RV-tip 212 to RV-coil 214 electrodes. If an RV-ring electrode ispresent, bipolar pacing may be accomplished by delivering the pacingpulses to the RV-tip 212 and RV-ring 211 electrodes.

Left ventricular pacing may be implemented using unipolar or bipolarconfigurations. Unipolar LV pacing may include, for example, pacingpulses delivered between the LV distal electrode 213 and the can 209.Alternatively, bipolar LV pacing may be accomplished by delivering thepacing pulses using the LV distal electrode 213 and the LV proximalelectrode 217.

Similarly, unipolar (RA-tip electrode 256 to can electrode 209) atrialpacing or bipolar (RA-tip electrode 256 to RA-ring electrode 254) atrialpacing may be provided by the CRM device 300.

The CRM device 300 may also provide tachyarrhythmia therapy. Forexample, tachyarrhythmia therapy may be provided in the form ofanti-tachycardia pacing (ATP) pulses delivered to the heart. The ATPpulses may involve a series of timed paces of programmable width andamplitude that are implemented to interrupt a tachyarrhythmia episode.The ATP therapy may involve, for example, burst pacing at about 25 Hz toabout 50 Hz. In various implementations, the pace-to-pace interval mayhave a variable or constant length. For immediately life threateningarrhythmias, such as ventricular fibrillation, the therapy controlcircuitry 322 may control the delivery of one or a series ofdefibrillation shocks to the heart to terminate the fibrillation.

In the embodiment depicted in FIG. 3, electrodes RA-tip 256, RA-ring254, RV-tip 212, RV-ring 211, RV-coil 214, SVC coil 216, LV distalelectrode 213, LV proximal electrode 217, and can 209 are coupledthrough a switching matrix 310 to various sensing circuits 331, 332,335, 336. A right atrial sensing channel circuit 331 serves to sense andamplify electrical signals from the right atrium of the heart. Forexample, bipolar sensing in the right atrium may be implemented bysensing signals developed between the RA-tip 256 and RA-ring 254electrodes. The switch matrix 310 may be operated to couple the RA-tip256 and RA-ring 254 electrodes to the RA sensing channel circuit 331 toeffect bipolar sensing of right atrial signals. Alternatively, unipolarright atrial sensing may be accomplished by operating the switch matrix310 to couple the RA-tip 256 and can 209 electrodes to the RA sensingchannel circuit 331.

Cardiac signals sensed through the use of the RV-tip electrode 212 andRV-coil 214 or RV-ring electrode 211 are right ventricular (RV)near-field signals and are referred to as RV rate channel signalsherein. Bipolar rate channel sensing may be accomplished by operatingthe switch matrix 310 to couple the RV-tip electrode 212 and the RV-coil214 electrode or the RV-ring electrode 211 through the RV rate channelsensing circuitry 335. The rate channel signal may be detected, forexample, as a voltage developed between the RV-tip electrode 212 and theRV-coil 214 electrode or the RV-ring electrode 211. The RV rate channelsensing circuitry 335 serves to sense and amplify the RV rate channelsignal.

Unipolar RV sensing may be implemented, for example, by coupling theRV-tip 212 and can 209 electrodes to the RV rate channel sensingcircuitry 335. In this configuration, the rate channel signal isdetected as a voltage developed between the RV-tip 212 to can 209sensing vector.

The RV lead system may also include an RV-ring electrode 211 used forbipolar pacing and sensing. If an RV-ring electrode is included in thelead system, bipolar sensing may be accomplished by sensing a voltagedeveloped between the RV-tip 212 and RV-ring 211 electrodes.

Far-field signals, such as cardiac signals sensed through use ofdefibrillation coils or electrodes 214, 216, 209, are referred to asmorphology or shock channel signals herein. The shock channel signal maybe detected as a voltage developed between the RV-coil 214 to the canelectrode 209, the RV-coil 214 to the SVC-coil 216, or the RV-coil 214to the can electrode 209 shorted to the SVC-coil 216. The switch matrix310 is operated to couple the desired shock channel sensing vector,e.g., RV-coil to can, to the right ventricular shock channel sensingcircuitry 332. The RV shock channel sensing circuitry 332 serves tosense and amplify the shock channel signal.

The outputs of the switching matrix 310 may also be operated to coupleselected combinations of the electrodes to LV sensing channel circuitry336 for sensing electrical activity of the left ventricle. Bipolar leftventricular sensing may be accomplished by operating the switch matrix310 to couple the LV-distal 213 and the LV proximal electrodes 217through the LV channel sensing circuitry 336. In this configuration, theLV signal is detected as a voltage developed between the LV proximal andLV distal electrodes.

Unipolar LV sensing may be implemented, for example, by coupling the LVdistal 213 and can 209 electrodes to the LV sensing circuitry 336. Inthis configuration, the LV signal is detected as a voltage developedbetween the RV-tip 212 to can 209 sensing vector.

The CRM device 300 includes a plurality of independent rhythmdiscriminators 361, 362 and an arrhythmia classification processor 321coupled to the rhythm discriminators and configured to classify avariety of cardiac rhythms, including ventricular or atrial arrhythmias.The arrhythmia classification processor 321 may detect and/or classifyarrhythmias using the results from the plurality of independentdiscriminators 361, 362. The arrhythmia classification processorevaluates the results of the independent discriminators 361, 362 anddetermines if modification of the classification process is likely toenhance classification of the rhythm. If so, the arrhythmiaclassification processor modifies the process and reclassifies thecardiac rhythm based on the modified process. Modification of theprocess may involve further evaluation of the cardiac rhythm using theindependent rhythm discriminators 361, 362, or other processes.

The rhythm discriminators 361, 362 may include, for example, one or morediscriminators that operate by analyzing the heart rate of atrial orventricular chambers. It will be appreciated that heart rate may beevaluated by evaluating the intervals between successive cardiac events,such as A-A intervals (intervals between successive atrial events), V-Vintervals (intervals between successive ventricular events), A-Vintervals (intervals between an atrial event and a subsequentventricular event) and/or V-A intervals (intervals between a ventricularevent and a subsequent atrial event).

One or more rhythm discriminators may utilize intervals betweensuccessive cardiac events to evaluate, for example, the onset of acardiac arrhythmic episode, the stability of the atrial and/orventricular rhythms, and/or the duration of an arrhythmic episode.

One or more rhythm discriminators may evaluate the morphology ofindividual cardiac beat signals of the arrhythmic episode to classifythe arrhythmia. Cardiac beats associated with arrhythmias originatingthe in the atria may be discerned from cardiac beats associated witharrhythmias originating in the ventricles based on the morphologycharacteristics of the cardiac beat signals.

A morphology-based discriminator may compare the cardiac beat signals toone or more morphology templates. The morphology templates characterizecardiac beat signals that are representative of a particular type ofrhythm. In one implementation, the morphology-based discriminator maycompare one or more cardiac beat signals of an arrhythmia episode to amorphology template characterizing a supraventricular rhythm (SVR). Ifthe cardiac beat signals are not similar to the SVR template, then thearrhythmic episode is recognized as an arrhythmia of ventricular origin.If the cardiac beat signals are similar to the SVR template, then thearrhythmic episode is recognized as an arrhythmia of non-ventricularorigin.

If the results of the independent rhythm discriminators 361, 362conflict, are indeterminate, or are borderline, the arrhythmiaclassification processor 321 may operate to implement modifications tothe classification process to enhance classification.

Based on the arrhythmia classification determined by the arrhythmiaclassification processor 321, the therapy control may initiate anappropriate therapy, to terminate or mitigate the arrhythmia. Thetherapy may include ATP, cardioversion and/or defibrillation shocks, forexample. For some types of arrhythmia, therapy may not be appropriate,in which case therapy may be withheld.

The CRM device 300 may incorporate one or more metabolic sensors 345 forsensing the activity and/or hemodynamic need of the patient.Rate-adaptive pacemakers typically utilize metabolic sensors to adaptthe pacing rate to match the patient's hemodynamic need. A rate-adaptivepacing system may use an activity or respiration sensor to determine anappropriate pacing rate. Patient activity may be sensed, for example,using an accelerometer disposed within the housing of the pulsegenerator. Transthoracic impedance, which may be measured, for example,via the intracardiac electrodes, may be used to determine respirationrate. Sensor information from the metabolic sensor is used to adjust thepacing rate to support the patient's hemodynamic need. If the sensorsindicate the patient's activity and/or respiration rate is high, thenthe patient's pacing rate is increased to correspond to the level ofactivity or rate of respiration.

FIG. 4 is a flow diagram illustrating a method of classifying a cardiacrhythm in accordance with embodiments of the invention. The systeminitially classifies 411, 412, 413 the cardiac rhythm using a pluralityof independent rhythm discriminators. For example, the independentdiscriminators used in the initial rhythm classification process mayanalyze cardiac rates, interval patterns and/or the morphology ofcardiac signals associated with one or more beats of the arrhythmicepisode.

Following the initial classification by the independent discriminators411, 412, 413, the system may implement 420 rhythm classificationenhancement. If the rhythm classification is not enhanced, then thecardiac rhythm is classified 421 based on the results of the initialrhythm discriminators 411, 412, 413. If the rhythm classificationenhancement is implemented 420, then one or more additional rhythmdiscrimination procedures 431, 432, 433 may be utilized to enhanceclassification of the cardiac rhythm.

For example, the system may implement 420 rhythm classificationenhancement if two or more of the initial rhythm discriminators 411,412, 413 produce conflicting results. A conflict situation may arise,for example, if a first rhythm discriminator classifies the cardiacrhythm as an SVT and a second rhythm discriminator classifies thecardiac rhythm as a VT.

In another example, the system may implement 420 rhythm classificationenhancement if one or more of the initial discriminators 411, 412, 413are unable to classify the cardiac rhythm. In this situation, theinitial rhythm discriminators 411, 412, 413 may return a resultindicating that the cardiac rhythm is unknown or is unclassifiable.

In yet another example, the system may implement 420 rhythmclassification enhancement if one or more of the initial discriminatorsclassify the cardiac rhythm, but the classification is within aborderline range. For example, one or more of the initial discriminatorsmay classify the rhythm as one type of arrhythmia, e.g., SVT, but theclassification is within a borderline range of being classified as a VT.In this situation, a more definitive classification may be possible byperforming one or more of the additional discrimination procedures 432,432, 433 and reclassifying 434 the cardiac rhythm.

FIG. 4 illustrates three exemplary rhythm discrimination procedures thatmay be employed to enhance the rhythm classification. Otherdiscrimination procedures are possible, and the invention is not limitedto the three examples illustrated in FIG. 4. Additional discriminationprocedures used to enhance rhythm classification may involve, forexample, acquiring additional information 431 from other CRM devicefunctions. Cardiac rhythm management devices are often equipped with anumber of sensors that may be used for diagnostic or therapeuticpurposes. Sensor information acquired from such sensor components may beused in connection with cardiac rhythm classification.

In one configuration, a CRM device may incorporate an activity sensorthat generates a signal corresponding to the patient's level ofactivity. The sensor signal may be used by the CRM device to deliver ahemodynamically appropriate pacing therapy to the patient. Informationfrom the activity sensor and/or other sensors or systems of the CRMdevice may be utilized to enhance rhythm classification such as bydiscriminating between a physiologic sinus tachycardia and a pathologicarrhythmia. For example, if the patient's heart rate is relatively highand the activity sensor output indicates that the patient is also veryactive, then the rhythm discriminator 431 may classify the cardiacrhythm as physiologic sinus tachycardia caused by elevated patientactivity.

The additional discrimination procedures employed for enhanced rhythmclassification may involve 432 modifying a parameter of one or more ofthe initial rhythm discriminators 411-413 initially used for the rhythmclassification. Alternatively or additionally, one or more supplementarydiscriminators may be utilized 433 for rhythm classification. Followingimplementation of the additional discriminators 431, 432, 433, thecardiac rhythm is reclassified 434 based on the information from theinitial discriminators 411, 412, 413, information from other devicefunctions 431, information from the modified discriminators 432, and/orinformation from additional discrimination procedures 433.

The rhythm classification enhancements 431, 432, 433 may be used aloneor in combination. For example, the device may implement one of therhythm classification enhancements 431, 432, 433 before reclassifyingthe cardiac rhythm 434, or may implement a plurality of the rhythmclassification enhancements 431, 432, 433 before reclassification 434.If a plurality of the rhythm classification enhancements 431,432, 433are implemented, then the device may implement the rhythm classificationenhancements 431, 432, 433 in parallel or serially.

FIG. 5A is a flowchart illustrating a method of rhythm classification inaccordance with embodiments of the invention. In this embodiment, twoindependent discriminators are initially used to classify the cardiacrhythm. One initial discriminator comprises an interval-baseddiscriminator that utilizes the relationship between the atrial, rateand the ventricular rate to classify the cardiac rhythm. Another initialdiscriminator comprises a morphology-based discriminator that evaluatesthe morphology of the cardiac beats of the arrhythmic episode for rhythmclassification.

In accordance with this embodiment, the device measures 505 theventricular rate and the atrial rate of an arrhythmic episode. Forexample the ventricular and atrial rates may be expressed in terms ofaverage rates of a number of cardiac beats of the arrhythmic episode.The morphology of the beats of the arrhythmic episode is determined 515.

The rhythm is classified 525 using an interval-based process. The rhythmis classified 535 using a morphology-based process. If the results ofthe two initial rhythm classifications are not 542 in conflict andproduce definite results, then the cardiac rhythm may be classified 545based on the results of the initial discriminators. However, if theresults of the initial discriminators conflict or if one or both of theinitial discriminators produce indeterminate or borderline results 542,then the classification process may be modified 552 and the rhythmreclassified 555 using the modified process.

An independent discriminator may incorporate a multiplicity ofdiscrimination processes that may be used for rhythm discrimination. Forexample, an interval-based discriminator may examine one or more of therhythm onset, stability and/or the relationship between the atrial rate(A-rate) and the ventricular rate (V-rate) to classify the cardiacrhythm.

Comparison of the A-rate to the V-rate may provide insight into theorigin of the arrhythmia. For example, if the A-rate is greater than theV-rate, the origin of the rhythm is more likely to be atrial in origin.Thus, if the A-rate is greater than the V-rate, the rhythm may beclassified as SVT. However, if the V-rate rate is greater than theA-rate, then the origin of the arrhythmia is more likely ventricular inorigin and the rhythm may be classified as a VT. If there is roughly aone-to-one correspondence between the atrial and ventricular rates, thenthe system may not be able to classify the type of rhythm based the Arate and V rate relationship. However, classifications based solely onthe relationship between the atrial and ventricular rates may produceincorrect results, particularly if the difference between atrial andventricular rates is small.

In one configuration if the average rate of the last N, e.g. about 10,ventricular intervals is compared to the average rate of the last Natrial intervals. If the average ventricular rate is greater than theaverage atrial rate by at least a predetermined time such as about 10bpm, then the rhythm is determined to be a ventricular rhythm andappropriate therapy, such as ATP or defibrillation, may be delivered. Ifthe ventricular rate is less than the atrial rate, then therapy may bewithheld because the system determines that the rhythm is SVT.

Stability analysis may comprise an interval-based process used todistinguish unstable or irregular rhythms from stable rhythms. Stabilityanalysis may be accomplished by measuring the degree of variability inthe cardiac cycles, e.g., the V-V intervals between sensed R-waves. Thedegree of variability may allow the system to distinguish conductedatrial fibrillation (AF), which may produce greater V-V variability,from monomorphic VT, which is typically stable. Stability analysis mayalso be used to differentiate monomorphic VT from polymorphic VT.Distinguishing monomorphic VT from polymorphic VT may be used indetermining an appropriate treatment. For example, monomorphic VT may betreated using ATP, whereas polymorphic VT may be more successfullytreated using defibrillation shocks. Methods for recognizing andtreating various types of monomorphic VT are described in the followingcommonly owned U.S. patent application Ser. No. 10/955,831, filed Sep.30, 2004, Ser. No. 10/996,340, filed Nov. 23, 2004, Ser. No. 995,705,filed Nov. 23, 2004, and Ser. No. 10/995,655, filed Nov. 23, 2004 whichare incorporated herein by reference.

In one embodiment, the stability analysis algorithm calculates V-Vinterval differences and determines an average difference between V-Vintervals. After a period of time, which may correspond, for example, tothe duration interval discussed herein, the system evaluates the rhythmstability by comparing the current average difference to a stabilitythreshold. If the average difference is greater than the stabilitythreshold, the rhythm is determined to be unstable. Instability in thecardiac rhythm is an indication that the arrhythmia is atrial in origin(SVT).

The onset of the cardiac rhythm may be considered in classifying anarrhythmia. The onset criterion is particularly useful in discriminatingbetween sinus tachycardia and ventricular tachyarrhythmia. The onsetparameter of the cardiac rhythm represents a measurement of how quicklythe rhythm transitions from a slow rate to a faster rate. The onset ofthe rhythm may be examined to differentiate between physiologic sinustachycardias, which typically begin slowly, from pathologictachycardias, which typically begin suddenly. If the rate increase isgradual, the system may determine that a fast rhythm cardiac rhythm issinus tachycardia and withhold therapy. However, if the rate abruptlyincreases, it is more likely that the rhythm is a pathologic rhythm andthe system may deliver an appropriate therapy to terminate or mitigatethe arrhythmia.

The above rhythm discrimination techniques rely on examining theintervals between a number of cardiac beats to determine variousparameters of the cardiac rhythm including atrial and ventricular rates,onset, and/or stability. An alternative rhythm analysis may involveexamining the morphological characteristics of the electrical signalsassociated with one or more beats of the arrhythmic episode. In oneexample, various characteristics of a cardiac beat signal, e.g., numberof peaks, peak polarity, peak area, and/or sequence of feature pointsmay be compared to a template, such as a template representative of asupraventricular rhythm (SVR). If the morphology of one or more of thecardiac beat signals of the arrhythmic episode are consistent with theSVR template then the arrhythmia is determined to be SVT. However, ifone or more of the cardiac beat signals of the arrhythmic episode areinconsistent with the SVR template, then the arrhythmia is determined tobe VT.

A cardiac signal may be considered to be consistent with a template ifthe features, samples, or other morphological characteristics of thecardiac signal are determined to be sufficiently similar to thecorresponding template features, samples, or morphologicalcharacteristics. If a cardiac signal is sufficiently similar to atemplate representative of a particular type of cardiac beat, e.g., andSVR beat, then the cardiac signal may be classified as the particulartype of beat. Various techniques may be used to compare a template and acardiac signal, including the correlation techniques described herein.

Rhythm classification by morphology analysis may involve comparingfeatures points of one or more cardiac beat signals of an arrhythmicepisode to corresponding feature points of a template representative ofan SVR beat. In one implementation, acquisition and use of templates formorphology analysis may be accomplished using a two channel approach.Cardiac beats are sensed on ventricular rate channel and a ventricularshock channel. In this example, a feature of the rate channel signal,e.g., the rate channel R-wave peak, may be used as a fiducial point toalign the shock channels signals of multiple cardiac beats with themorphology template.

FIGS. 5B and 5C illustrate the process of shock signal alignment. FIG.5B illustrates aligned rate channel signals of a template (SVR) beat 510and a VT beat 520, respectively. FIG. 5C illustrates the aligned shockchannel signals of the template beat 530 and the VT beat 540.

The template may comprise a sequence of feature points of a shockchannel signal representative of a particular type of rhythm. Forexample, with reference to FIG. 5C, a number of feature points 550 maybe selected based as a template representative of the SVR beat. Samples560 of the aligned shock channel beat 540 are compared to correspondingtemplate samples 550. The cardiac beat may be recognized as the type ofcardiac rhythm represented by the template if the cardiac beat samplesare similar to the template samples.

The similarity between the cardiac beat signal and the template may becompared by determining the correlation between the cardiac beat signalsamples and the template features. For example, the correlation betweenthe template and a cardiac beat may be expressed in terms of a featurecorrelation coefficient (FCC). In one particular embodiment, Equation 1,provided below, is used to compute the FCC between the template featuresand the beat features.

$\begin{matrix}{{FCC} = \frac{\left( {{N{\sum\limits_{i = 1}^{N}{X_{i}Y_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}X_{i}} \right)\left( {\sum\limits_{i = 1}^{N}Y_{i}} \right)}} \right)^{2}}{\left( {{N{\sum\limits_{i = 1}^{N}X_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}X_{i}} \right)^{2}} \right)\left( {{N{\sum\limits_{i = 1}^{N}Y_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}Y_{i}} \right)^{2}} \right)}} & \lbrack 1\rbrack\end{matrix}$

where, Xi represents template N features and Yi represents beat Nfeatures, and N=8 in this illustrative example. The sign of thenumerator term is checked before squaring. If the numerator is negative,the beat is uncorrelated, and the remainder of the computation need notbe performed.

Alternatively, a generalized equation may be used for computation of acorrelation coefficient in accordance with a correlation waveformanalysis (CWA) technique. An equation for calculation of the correlationcoefficient (CC) using this technique is set forth in Equation 2.

$\begin{matrix}{{CC} = \frac{{N{\sum\limits_{i = 1}^{N}{X_{i}Y_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}X_{i}} \right)\left( {\sum\limits_{i = 1}^{N}Y_{i}} \right)}}{\sqrt{\left( {{N{\sum\limits_{i = 1}^{N}X_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}X_{i}} \right)^{2}} \right)\left( {{N{\sum\limits_{i = 1}^{N}Y_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}Y_{i}} \right)^{2}} \right)}}} & \lbrack 2\rbrack\end{matrix}$

where, Xi represents template N samples and Yi represents signal Nsamples in this illustrative example. Methods and systems involving theuse of a morphology template for determining cardiac rhythms, aspects ofwhich may be implemented by the embodiments discussed herein, aredescribed in commonly owned U.S. Pat. No. 6,449,503 which isincorporated herein by reference.

The system may examine the duration of the cardiac rhythm in theclassification process. If the arrhythmia is a non-sustained orself-terminating rhythm, then therapy delivery may be avoided. Forexample, the system may trigger a duration timer when a fast ventricularrhythm is detected. The duration timer may be programmable to time aninterval of about 1 second to about 60 seconds, for example. Eachcardiac cycle, the system checks for a timeout of the duration timer. Ifthe fast cardiac rate persists throughout the duration timer interval,then the duration requirement of the rhythm classification is met andthe system may initiate therapy.

As previously described, the relationship between the atrial rate andthe ventricular rate is particularly useful in discriminating betweenrhythms arising from the atria (SVT) and rhythms arising in theventricles (VT). The relationship between the A-rate and the V-rate maybe used as a starting point for classification purposes. FIG. 6A is agraph illustrating regions of the A-V rate plane that may be utilizedfor classification of the cardiac rhythm in accordance with embodimentsof the invention.

In one embodiment, cardiac rhythms that fall into one or more areas ofthe A-V rate plane may be classified based on the relationship betweenthe A rate and the V rate. If the V rate is less than a threshold value605 and if the V rate is greater than the A rate (Region 7), then therhythm may be classified as sinus rhythm or bradycardia. If the V rateis less than a threshold value 605 and if the A rate is greater than theV rate (Region 8), then the rhythm is classified as SVT. The system maybe configured such that rhythms falling into Regions 7 and 8 may notinitiate delivery of therapy. The flowcharts of FIGS. 6B, 7-11illustrate methods of classifying a cardiac rhythm that falls into oneof the Regions 1-6 in accordance with an embodiment of the invention.

As illustrated in the flowchart of FIG. 6B, the cardiac rhythm isclassified using a morphology-based rhythm discriminator 630 and aninterval-based discriminator 640. For example, the morphology-basedrhythm discriminator may involve comparing the cardiac beat signals to amorphology template representative of a supraventricular conductedrhythm (SVR) and determining the morphological similarity between thecardiac beat signal and the SVR template. If the cardiac signal issufficiently similar to the SVR template, then the rhythm is classifiedas SVT. If the cardiac signal is not sufficiently similar to the SVRtemplate, then the cardiac rhythm is classified as VT.

The interval-based discrimination procedure involves evaluating therelationship between the A-rate and the V-rate (V>A algorithm). If theV-rate is above a threshold value and the A-rate is sufficiently greaterthan the V-rate, the rhythm is classified as SVT. If the V-rate is abovethe threshold value and the V-rate is sufficiently greater than theA-rate, then the rhythm is classified as VT. If the A rate and the Vrate are about equal, then the result of the interval-baseddiscriminator is indeterminate.

After classification of the rhythm using the morphology-baseddiscriminator and the interval-based discriminator, the systemdetermines whether or not to modify 645 the classification process toenhance rhythm classification. The system may determine that rhythmclassification enhancement is not desired or required 650.

The system may determine to enhance rhythm classification, for example,if the results of the morphology- and interval-based rhythmdiscriminators conflict or are indeterminate. In these situations, theclassification process may be modified and the rhythm re-classifiedusing the modified classification process. If the system determines thatenhancing the classification process is not required or desired, thenthe cardiac rhythm is classified based on the initial results of themorphology-based discriminator and/or the interval-based discriminator.The system may determine that enhancement of the classification processis not required or desired, for example, if the classification resultsproduced by the morphology and interval-based discriminators are not inconflict and are not indeterminate.

Various optional methods 660, 670, 680 for modifying the classificationprocess when the rhythm corresponds to various areas of the A-V planeare illustrated in the flowcharts of FIGS. 7-11 which follow from FIG.6B. If the cardiac rhythm falls into 660 Regions 1-2 of the A-V rateplane, and the classification results of the morphology andinterval-based discriminators conflict, then any or all of the optionalprocess illustrated in the flowchart of FIG. 7 may be employed toresolve the conflict. The flowchart of FIG. 8 illustrates variousoptional processes that may be implemented if the cardiac rhythm fallsinto 670 Regions 3, 4, or 5 of the A-V rate plane. If the A rate and theV rate are about equal 680, then any or all of the optional processesillustrated in the flowcharts of FIG. 9, 10, or 11 may be used.

FIG. 7 illustrates various methods that may be employed to enhancecardiac rhythm classification when the cardiac rhythm falls into 710Region 1 or Region 2 of the A-V rate plane. As previously discussed, theinitial classification process, such as the process illustrated in FIG.6B, may involve the use of an interval-based discriminator and amorphology-based discriminator. A cardiac rhythm that falls into Regions1 or 2 may be initially classified 750 as VT by the interval-basedrhythm discriminator because the ventricular rate is sufficientlygreater than the atrial rate indicating a rhythm of ventricular origin.However, the morphology-based discriminator will indicate 760 SVT if themorphology of the cardiac signal is determined to be similar to an SVRtemplate. In this scenario, a conflict 770 exists between the results ofthe interval-based rhythm discriminator and the morphology-based rhythmdiscriminator.

The conflict may be resolved by implementing one or more supplementaldiscrimination procedures that modify the initial classificationprocess. Modification of the initial classification process may beaccomplished by modifying 781 one or more parameters of theinterval-based discriminator and/or the morphology-based discriminator,using additional discrimination procedures including evaluating rhythmonset and stability 782, 784, 786, and/or acquiring further informationby analyzing 788 undersensing of atrial events. The processes formodifying the initial classification process 781, 782, 784, 786, 788 maybe implemented in any order or may be implemented in parallel. In apreferred embodiment, modification of the V>A algorithm 781 isimplemented first, with one or more additional processes 782, 784, 786,788 optionally implemented if the rhythm remains unclassified. Followingmodification of the initial classification process by any of theprocedures 781, 782, 784, 786, 788, the rhythm is reclassified 790 usingthe modified classification process.

If the results of the initial interval-based and morphology-baseddiscriminators conflict, the initial interval-based discriminator (V>Aalgorithm) may be modified by changing one or more parameters of thealgorithm. For example, the V>A algorithm may be modified by modifyingthe threshold for determination of VT. In one scenario, the initialalgorithm may classify the rhythm as VT if the V-rate exceeds the A-rateby a predetermined threshold value, wherein the initial threshold valuemay be about 10 bpm. If the V-rate is substantially in excess of theA-rate, then the likelihood of VT is increased. The V>A algorithm may bemodified by modifying the threshold value for detection of VT, such asby increasing the threshold to a higher value than was initially used,e.g. to about 50 beats. If the V-rate exceeds the A-rate by the newthreshold value then the cardiac rhythm is reclassified as VT.

In another example of parameter modification 781, the intervals includedin the A-rate and/or V-rate calculation may be modified and therelationship between the A-rate and the V-rate recalculated based on themodified intervals. In one embodiment, the A-rate calculation may bemodified by changing the criteria for including A-A intervals used forthe A rate calculation. The V-rate calculation may be modified bychanging the criteria for including V-V intervals used for the V-ratecalculation. For example, the V-rate calculation criteria may bemodified by excluding one or more of the shortest V-V intervals out of apredetermined number of V-V intervals, e.g., excluding about 2 of theshortest V-V intervals out of about 10 intervals.

The A-rate calculation criteria may be modified by excluding one or moreof the longest A-A intervals out of a predetermined number of A-Aintervals, e.g., excluding about 2 of the longest A-A intervals out ofabout 10 intervals. The A-rate and/or V-rate is recalculated using themodified criteria and the relationship between the A-rate and the V-rateis determined. The system reclassifies 790 the cardiac rhythm based onthe modified A-rate and/or V-rate parameters. Methods and systems formodifying the V>A algorithm, aspects of which may be utilized in theembodiments discussed herein, is described in commonly owned U.S. patentapplication Ser. No. 10/862,779, filed Jun. 7, 2004, and incorporatedherein by reference.

If the results of the morphology and interval-based discriminatorsconflict 770, the conflict may be resolved using 782 additionaldiscrimination procedures and/or additional information. For example ifthe morphology-based discriminator indicates SVT and the interval-baseddiscriminator (V>A algorithm) indicates VT, the system may implement anadditional discrimination procedure that checks the onset of the cardiacrhythm. If the onset is gradual, mitigating toward an SVT rhythmclassification, information from other device processes may be used toconfirm SVT. For example, sensor information used for adapting thepacing rate of a cardiac rhythm device may be used in the rhythmdiscrimination process.

Rate-adaptive pacemakers typically incorporate metabolic sensors toadapt the pacing rate to match the patient's hemodynamic need. Arate-adaptive pacing system may use an activity or respiration sensor todetermine an appropriate pacing rate. Patient activity may be sensed,for example, using an accelerometer disposed within the housing of thepulse generator. Transthoracic impedance, which may be measured, forexample, via intracardiac electrodes, may be used to determinerespiration rate. Sensor information from the metabolic sensor is usedto adjust the pacing rate to support the patient's hemodynamic need. Ifthe sensors indicate the patient's activity and/or respiration rate ishigh, then the patient's pacing rate is increased to correspond to thelevel of activity or rate of respiration.

Sensor information from the pacing-rate adaptation process may beutilized in classifying a cardiac rhythm. For example, if the sensoroutput indicates relatively high patient activity or respiration rate,and the onset of the fast cardiac rhythm is gradual, then the system mayreclassify 790 the rhythm as SVT.

Another example of using additional rhythm discriminators is provided atblock 786. If the interval-based discriminator (V>A algorithm) indicatesVT and the morphology-based discriminator indicates SVT, then anadditional discrimination process may be implemented involving assessingthe stability of the cardiac rhythm. A more stable ventricular rhythm isassociated with VT, whereas a less stable rhythm is associated with SVT.

Stability analysis may be accomplished by measuring the degree ofvariability in the cardiac cycles, e.g., the V-V intervals betweensensed R-waves. The degree of variability may allow the system todistinguish conducted atrial fibrillation (SVT), which may producegreater V-V variability, from monomorphic VT, which is typically stable.Stability analysis may also be used to differentiate monomorphic VT frompolymorphic VT. Distinguishing monomorphic VT from polymorphic VT may beused in determining an appropriate treatment. For example, monomorphicVT may be treated using ATP, whereas polymorphic VT may be moresuccessfully treated using defibrillation shocks.

The rhythm is reclassified 790 after assessing 786 the stability of therhythm. If the rhythm is sufficiently stable, the system may reclassifythe rhythm as VT.

Conflict between the interval-based and morphology-based discriminators770 may be resolved using additional discrimination processes inaddition to modification of an initially used discrimination process tofurther evaluate the cardiac rhythm. For example, as indicated at block784, the system may utilize an additional discriminator (onset) and maymodify a parameter (duration window size) of the V>A algorithm. Therhythm is reclassified 790 using the additional and modifieddiscriminators.

If the onset of the rhythm is gradual, mitigating toward aclassification of SVT, then the system may determine if the VT rhythmclassification indicated by the V>A algorithm is sustained. Determiningif the VT classification is sustained may be accomplished byreevaluating the relationship between the A rate and the V rate. Thereevaluation may involve the same evaluation window size as wasinitially used, or an increased evaluation window size for measuring theA-A intervals and V-V intervals for determining the relationship betweenthe A-rate and the V-rate.

For example, if the initially used window size is 10 intervals, the A-Aand V-V intervals may be reassessed in a new window of 10 intervals.Alternatively, the window size for measuring the A-A and V-V intervalsmay be increased. In one implementation, the initially used window sizemay be about 10 intervals and the increased window size may be about 20to about 40 intervals. The relationship between the A-rate and theV-rate is reevaluated using intervals measured during the new ormodified window. The cardiac rhythm is reclassified 790 based oninformation from the onset discriminator and the V>A algorithm based onintervals measured during the new or modified evaluation window.

Another example of using additional information to resolve the conflictbetween discriminators involves determining 788 if atrial events areundersensed. Atrial events may be undersensed if the atrial rate is highcausing atrial events to occur during a blanking period, e.g., across-chamber atrial blanking period following a sensed or pacedventricular event. Atrial events occurring during the blanking periodare not sensed and thus are not included in the A rate estimation forthe V>A algorithm. Undersensing of atrial events may be analyzed byreassessing atrial electrogram (EGM) data collected during thearrhythmic episode. The system may review the collected atrial data todetermine if atrial events were present during the blanking period.

Analyzing the cardiac rhythm to find undersensed atrial events requiresdiscriminating P-waves associated with intrinsic atrial depolarizationsfrom far field R waves (FFRW). Far field R waves ventriculardepolarizations sensed on the atrial channel. Far field R waves may bedistinguished from legitimate P waves based on the relative peak valuesof the signal waveforms respectively associated with the two differenttypes of waves. Legitimate P-waves have a different morphology thanFFRWs and typically exhibit a larger peak amplitude than FFRWs.

In one implementation, the device may discriminate between legitimateP-waves and FFRW by comparing the peak amplitude of signals sensedduring a blanking period to a value associated with FFRW. If the peakamplitude exceeds the threshold value, the signal is assumed to be alegitimate P-wave.

In another implementation, the device may form a template based onP-wave morphology. After formation of the P-wave template, sensed atrialsignals are compared to the P-wave template, for example, by calculatinga correlation coefficient using Equations 1 or 2. If the sensed atrialsignals are similar to the P-wave template morphology, then the devicedetermines that the sensed signals are legitimate P-waves. In anotherimplementation, the device may form a FFRW template using signals sensedduring a atrial blanking period following a ventricular event. If atrialsignals sensed after formation of the FFRW template are similar to thetemplate, then they are recognized as FFRWs. If the subsequently sensedsignals are not similar to the template, then they are recognized aslegitimate P-waves.

The use of a wide bandwidth amplifier may be helpful in discriminatinglegitimate P-waves from FFRWs. In devices that are equipped with a wideband amplifier and are capable of selecting electrode combinations for aparticular channel, the atrial electrodes may be coupled to the wideband amplifier for detection of legitimate P-waves.

The newly uncovered P waves may be used in the A-rate estimation of theV>A algorithm. The cardiac rhythm is reclassified 790 based on themodified V>A algorithm which uses the uncovered P-waves.

The flowchart of FIG. 8 illustrates various optional procedures 882,884, 885, 886, 888 that may be employed to enhance cardiac rhythmclassification when the cardiac rhythm falls into 810 Regions 3, 4, or 5of the A-V rate plane. As previously discussed, the initialclassification process may involve the use of an interval-baseddiscriminator and a morphology-based discriminator. A cardiac rhythmthat falls into Regions 3, 4 or 5 may be initially classified 850 as SVTby the interval-based rhythm discriminator because the atrial rate isgreater than the ventricular rate indicating a rhythm of atrial origin.However, the morphology-based discriminator will classify 860 the rhythmas VT if the morphology of the cardiac signal is not correlated to theSVR template. In this scenario, a conflict 870 exists between theresults of the interval-based rhythm discriminator and themorphology-based rhythm discriminator.

The conflict 870 may be resolved by modifying the initial classificationprocess. Modification of the initial classification process may beaccomplished, for example, by modifying one or more parameters of theinterval-based discriminator and/or the morphology-based discriminatorand reclassifying the cardiac rhythm using the modified discriminators.Alternatively or additionally, one or more supplemental discriminationprocedures may be used to classify the cardiac rhythm. Further,additional information from other cardiac processes may be used fordetermining the cardiac rhythm. Following modification of the initialclassification process by any of the optional procedures 882, 884, 885,886, 888, the rhythm is reclassified 890 using the modifiedclassification process.

One example of the use of supplemental discriminators for classificationof the cardiac rhythm is provided at block 882. If the interval-baseddiscriminator (V>A algorithm) indicates SVT and the morphology-baseddiscriminator indicates VT, then one or more supplemental discriminationprocesses may be implemented. The additional processes may involve, forexample, assessing the stability of the cardiac rhythm, assessing theonset of the cardiac rhythm, assessing both the stability and the onset,or using other discrimination techniques as described below.

Rhythm stability evaluation may be useful in classifying cardiac rhythmsbecause more stable ventricular rhythms are most often associated withmonomorphic VT, whereas less stable rhythms are most often associatedwith AF. As previously discussed, stability analysis may be accomplishedby measuring the degree of variability in the cardiac cycles, e.g., theV-V intervals between sensed R-waves.

As previously discussed, stability evaluation may be implemented, forexample, by measuring the difference in intervals measured betweenventricular events (V-V intervals). If the difference between the V-Vintervals is less than a threshold value, then the rhythm may bedetermined to be stable. If the difference between the V-V intervals isgreater than the threshold value, then the rhythm may be determined tobe unstable. The degree of variability in V-V intervals may allow thesystem to distinguish conducted atrial tachyarrhythmia (SVT), which mayproduce greater V-V variability, from monomorphic VT, which typicallyexhibits a more stable ventricular rhythm.

The onset of the cardiac rhythm may additionally or alternatively beevaluated and used as a supplemental discriminator for classifying thecardiac rhythm. As previously discussed, onset criteria is useful indiscriminating between sinus tachycardia and ventriculartachyarrhythmia. The onset parameter of the cardiac rhythm represents ameasurement of how quickly the rhythm transitions from a slower rate toa faster rate. The onset of the rhythm may be examined to differentiatebetween physiologic sinus tachycardias, which typically begin slowly,from pathologic tachycardias, which typically begin suddenly. If therate increase is gradual, the system may determine that a fast rhythmcardiac rhythm is sinus tachycardia and withhold therapy. However, ifthe rate abruptly increases, it is more likely that the rhythm is apathologic rhythm and the system may deliver an appropriate therapy toterminate or mitigate the arrhythmia.

In an embodiment which exemplifies the use of 882 a supplementaldiscriminator along with a modification of the V>A discriminator, thesystem determines if the A-rate is greater than a threshold value. Ifso, the system may also evaluate the rhythm stability beforereclassifying the rhythm. For example, if the A rate is greater than athreshold value, e.g. about 200 bpm, and the rhythm is unstable, thenthe system may classify the rhythm as SVT. After the rhythm is evaluated882 using the supplemental discriminator and the modified V>Adiscriminator, the rhythm is reclassified 890.

In another embodiment, which exemplifies the use of multiplesupplemental discriminators, the system may assess 888 both onset andstability discriminators. If the rhythm is stable and exhibits suddenonset, then the likelihood of atrial flutter (AFL) or atrial tachycardia(AT) is increased. The system may initiate procedures that discriminatepotentially pace terminable atrial rhythms, such as AFL and AT fromthose that are more amenable to shock therapy, such as atrialfibrillation. Instead of comparing the A-rate to a threshold value, suchas a threshold value associated with atrial fibrillation, the system mayconsider three elements, for example, a range statistic, a minimuminterval and a dispersion statistic derived from a set of depolarizationintervals. In one implementation, the range statistic comprises thedifference between the largest and the smallest depolarization intervalof the set, the minimum interval comprises the smallest interval, andthe dispersion comprises the standard deviation of the intervals. Thethree elements may be compared to a threshold to identify a rhythm asatrial flutter or atrial fibrillation. Methods and systems foridentifying rhythms, aspects of which may be incorporated into theembodiments described herein, are discussed in commonly owned U.S. Pat.No. 6,681,134 which is incorporated herein by reference. After therhythm is evaluated 888 using the multiple supplemental discriminators,the rhythm is reclassified 890.

As previously described, conflict in the results of the interval-baseddiscriminator and the morphology based discriminator may be resolved bymodifying a parameter of one or both of the initial discriminators. Anexample of using a supplemental discriminator along with modifying aparameter of an initially used discriminator is illustrated at block884. If the interval-based discriminator classifies the rhythm as SVTand the morphology-based discriminator classifies VT, then thecorrelation threshold for the morphology-based discriminator may belowered 884. For example, the correlation coefficient may be loweredfrom about 0.94 to about 0.80. The rhythm may be reassessed using themodified correlation threshold. The stability of the rhythm may also beevaluated. After the rhythm is evaluated 884 based on stability and themodified correlation coefficient, the rhythm is reclassified 890.

Another example of using a modified initial discriminator is illustratedat Block 886. In this example, a different template may be used in placeof the morphology discriminator template used in the initialclassification. For example, the initial morphology template mayrepresent morphology template associated with a resting state of thepatient. The system may use a template associated with a differentsupraventricular rhythm. In one implementation, the system maysubstitute a template representative of bundle branch block (BBB) or atemplate that is representative of the patient's SVR during a period ofincreased activity or metabolic need.

The cardiac signal of one or more cardiac beats of the arrhythmicepisode may be compared to the substituted template and a determinationmade as to whether a correlation exists between the rhythm and thetemplate. In one example, the template used may be selected based on theactivity level or metabolic need indicated by the systemmetabolic/activity sensor(s). The use of templates associated withvarious metabolic states of the patient may produce increasedspecificity in classifying the cardiac rhythm. Methods and systems forusing multiple types of templates for classifying cardiac rhythms aredescribed in commonly owned U.S. patent application Ser. No. 10/291,200filed Nov. 8, 2002 and incorporated herein by reference. After therhythm is evaluated 886 using the substituted template, the rhythm isreclassified 890.

Modification of the initial classification process may involve blending885 the results of two or more initial or supplemental discriminators.In one example, the system may use the classification results of themorphology discriminator along with a rhythm stability discriminator.Marginally correlated rhythms may result in a higher weighting onstability. For example, if the correlation threshold for themorphology-based discriminator is about 0.9 to detect SVT, a marginallycorrelated rhythm may comprise a rhythm with a correlation factor ofabout 0.8. In this scenario, the system may evaluate the rhythmstability prior to reclassifying 890 the rhythm.

The conflict may be resolved by combining or blending 885 informationfrom the initial rhythm discriminators, modified rhythm discriminators,and/or supplemental rhythm discriminators. Blending the information fromthe initial, modified, and/or supplemental discriminators may involvedetermining a weighting factor for each discriminator result. Thecardiac rhythm is reclassified 890 based on a combination of theweighted results.

Blending 885 information from the initial, modified and/or supplementalrhythm discriminators may involve calculating a parameter of one rhythmdiscriminator as a linear or non-linear function of a parameter ofanother rhythm discriminator. In one embodiment, illustrated in thegraph of FIG. 8B, a coefficient threshold used for morphology-basedrhythm discrimination may be calculated as a linear function of thestability parameter. In other implementations, the stability parametermay be calculated based on the coefficient threshold.

The scenario involving computing one discrimination parameter as afunction of another parameter provides a relatively simple example ofblending rhythm discrimination processes. Another aspect of theinvention involves blending discrimination parameters in a threedimensional or greater system. In one example, a particulardiscriminator parameter may be determined as function of any number ofother discriminator parameters.

The processes 882, 884, 885, 886, 888 for modifying the initialclassification process may be implemented in any order or may beimplemented in parallel. In one implementation, the system may firstexamine the stability 882 of the A-rate and determine if the rateexceeds a threshold to identify the rhythm. Additional rhythmidentification processes 884, 885, 886, 888 may be implemented if therhythm cannot be identified from the A-rate stability and rateinformation.

In another implementation, a lower correlation threshold may be used 884to determine if correlation with an SVR template is achieved using thelower correlation threshold. If the rhythm morphology is consistent withthe SVR template using the lower correlation threshold, then the rhythmmay be classified 890 as SVT. If the rhythm classification is stillindeterminate, one or more rhythm identification processes 882, 885,886, 888 may be employed.

In yet another implementation, the A-rate stability and rate may bechecked 882, and the morphology may be evaluated using a lowercorrelation threshold 884. If neither of these processes yields adefinitive rhythm identification, then the system may evaluate themorphology of the rhythm using a different type of template 886.

Blending information from rhythm discriminators may involve blending theresults of the rhythm discriminators in multi-dimensional discriminationspace to classify the rhythm. Methods for implementing blending resultsfrom discriminators are described in commonly owned U.S. Pat. No.6,681,134.

The flowcharts of FIG. 9-11 illustrate various procedures 960, 982, 986,988, 1060, 1082, 1086, 1088, 1160, 1182, 1183, 1184, 1185, 1186 that maybe employed to enhance cardiac rhythm classification when the A-rate andthe V-rate are about equal 910, 1010, 1110. As in the embodimentpreviously discussed, the initial classification process may involve theuse of an interval-based discriminator (V>A algorithm) and amorphology-based discriminator. A cardiac rhythm that falls into Regions6 (A rate about equal to V rate) may be initially classifiedindeterminate by the interval-based rhythm discriminator. In thisscenario, the interval-based discriminator cannot determine 930, 1030,1130 the type of rhythm (classification is indeterminate). Themorphology based discriminator may classify 950, 1050, 1150 the rhythmas SVT or VT. The rhythm classification may be enhanced 940, 1040, 1140using one or more of the optional processes 960, 982, 986, 988, 1060,1082, 1086, 1088, 1160, 1182, 1183, 1184, 1185, 1186. Followingimplementation of one or more of the optional processes 960, 982, 986,988, 1060, 1082, 1086, 1088, 1160, 1182, 1183, 1184, 1185, 1186, thecardiac rhythm may be reclassified 990, 1090, 1190.

In the embodiments illustrated in FIGS. 9-11, the system use onset andstability discriminators to enhance the initial classification process.The flowchart of FIG. 9 illustrates optional processes 982, 986, 988that may be employed for rhythm classification enhancement if the rhythmonset is gradual and the rhythm is stable 960. Preferably, one or bothof processes 982 or 986 are implemented first with addition processesimplemented if the rhythm is still unclassified. Following modificationof the initial classification process by at least one of the optionalprocedures 982, 986, 988, the rhythm is reclassified 990 using themodified classification process.

The cardiac rhythm classification may be enhanced, for example, bymodifying a parameter of one or both of the initial discriminators. Forexample, if the morphology-based discriminator classified the rhythm asVT during the initial classification, the correlation coefficient forthe morphology-based discriminator may be lowered 986. In oneimplementation, the correlation coefficient is lowered from about 0.94to about 0.8. The rhythm may be reassessed using the modifiedcorrelation coefficient. If the morphology of the cardiac beats of therhythm are correlated to the SVR template using the decreasedcorrelation coefficient, then the rhythm may be reclassified as SVT. Ifthe morphology of the cardiac beats is uncorrelated to the SVR template,then the rhythm may be reclassified as VT.

Another example of modifying a parameter of an initial discriminator isillustrated at Block 982. In this example, a different template may beused in place of the morphology-based discriminator template used in theinitial classification. In one scenario, the initial morphology templatemay represent morphology template associated with a resting state of thepatient. The system may substitute a different template forclassification, such as a template associated with a sinus tachycardia(ST) or other type of supraventricular rhythm. The ST template may beacquired during a sinus tachycardia episode, or may be generated basedon a stored sinus tachycardia episode.

The cardiac signal may be compared to the substituted template and adetermination made as to whether a correlation exists between thecardiac beats of the rhythm and the template. If the rhythm beats arecorrelated to the ST template, then the rhythm may be reclassified 990as SVT.

Yet another example of modifying a parameter of an initial discriminatoris illustrated at Block 988. In this example, the duration that therhythm is evaluated may be increased to determine if the rhythm is asustained rhythm.

In one implementation, the duration period that the rhythm is assessedmay be increased to about 10 to about 20 seconds. The rhythm may bereclassified 990 using one or both of the initial rhythm discriminatorsbased on information acquired during the increased duration period.

The flowchart of FIG. 10 illustrates optional processes 1082, 1086, 1088that may be employed for rhythm classification if the rhythm onset issudden and the rhythm is stable 1060. For example, process 1082 may befirst applied followed by additional processes if required or desired.Following modification of the initial classification process by one ormore of the optional procedures 1082, 1086, 1088, the rhythm isreclassified 1090 using the modified classification process.

Cardiac rhythm classification may be enhanced by examining 1082 theonset pattern 1082 and/or the timing or morphology 1086 of the atrialbeats. In one implementation, classification of the cardiac rhythm mayinvolve a set of rules based on onset pattern 1082 and atrial channelmorphology 1086. One example of such a set of rules is that may be usedfor cardiac rhythm classification enhancement is as follows:

RULE 1. If the onset pattern is A1-V1-V2-A2 and the morphology orcharacteristics of A1 are different from the morphology orcharacteristics of A2 as detected on the atrial channel then the rhythmis likely to be VT. A rhythm that is correlated to the SVR templateusing the initial morphology-based discriminator may be further analyzedbefore classifying the rhythm as VT. In one embodiment, AA, AV, VA,and/or VV interval analysis is implemented to confirm the rhythm is VT.

RULE 2. If the onset pattern is A1-V1-A2-V2 and the morphology orcharacteristics of A1 and A2 are unchanged as detected on the atrialchannel, then a rhythm that is uncorrelated to the SVR template usingthe initial morphology-based discriminator but correlated using alowered FCC threshold or secondary template is classified as SVT.

RULE 3. If the onset pattern is A1-V1-A2-V2 or A1-A2-V1-A3-V2 and themorphology or characteristics of the atrial beats A1, A2, A3 change frombeat to beat as detected on the atrial channel, then a rhythm that isuncorrelated to the SVR template using the initial morphology-baseddiscriminator but correlated using a lowered FCC or secondary templateis classified as SVT.

The above rules represent one possible set of rules that may be utilizedin connection with classifying the cardiac rhythm. Other or additionalrules may alternatively be used. For example, the rules may be basedonly on onset pattern or only on atrial morphology.

Cardiac rhythm classification may be enhanced by examining 1088 thevariation of various intervals, including A-A intervals A-V intervals,V-A intervals and/or V-V intervals. SVT and VT rhythms may be identifiedbased upon the relative variability of the VA and AV intervals. Forexample, if the atrial rate is about equal to the ventricular rate, anSVT may be detected if the VA interval variability exceeds the AVinterval variability. A VT may be detected if the AV intervalvariability exceeds the VA interval variability. Methods and systems foridentifying cardiac rhythms based on evaluating pairing of atrial andventricular intervals and/or interval variability, aspects of which maybe utilized in the embodiments described herein, are discussed incommonly owned U.S. Pat. No. 6,522,917 and U.S. patent application Ser.No. 09/982,116, filed Oct. 17, 2001, which are incorporated herein byreference.

The flowchart of FIG. 11 illustrates optional processes 1182, 1183,1184, 1185, 1186 that may be employed for rhythm classificationenhancement. The optional processes 1182, 1183, 1184, 1185, 1186 of FIG.11 may be utilized if the A-rate is about equal to the V-rate 1110, theresults of the V>A discriminator are indeterminate 1130, the rhythm isclassified 1150 by the morphology discriminator as VT or SVT, and therhythm is unstable 1160. If the A-rate is about equal to the V-rate1110, the rhythm is unstable and uncorrelated to the SVR template, thereis a possibility that the rhythm is SVT. The processes 1182, 1183, 1184,1185, 1186 may be used to enhance 1140 the discrimination between VT andSVT rhythms. Following modification of the initial classificationprocess by any of the optional procedures 1182, 1183, 1184, 1185, 1186,the rhythm is reclassified 1190 using the modified classificationprocess.

Cardiac rhythm classification may be accomplished 1183 using a set ofrules involving the onset pattern and the characteristics or morphologyas previously described in connection with block 1082 of FIG. 10 above.Alternatively or additionally, the rhythm classification may involvedetermining 1184 interval variations as described previously inconnection with block 1088.

The correlation coefficient may be modified 1185 as described inconnection with 884 of FIG. 8. For example, the feature correlationcoefficient calculated using Equation 1 may be decreased from about 0.94to about 0.8. If the rhythm morphology is correlated to the templateusing the decreased correlation coefficient, then the rhythm may bereclassified 1190 as SVT.

In one example, the factors used in the V>A algorithm, e.g., the A-rateand/or V-rate may be modified 1182 to determine if the A-rate and V-rateare biased toward A>V or V>A. In one implementation, the A-rate and/orV-rate may be redetermined using a new criteria for including beats inthe A-rate and/or V-rate calculation. As previously described inconnection with block 781 of FIG. 7, the intervals included in theA-rate and/or V-rate calculation may be modified and the relationshipbetween the A rate and the V rate recalculated using the newestimations. In one embodiment, the A rate estimation may be modified bychanging the criteria for including A-A intervals used for the A ratecalculation. The V-rate estimation may be modified by changing thecriteria for including V-V intervals used for the V rate calculation.For example, the V rate estimation criteria may be modified by excludingone or more of the shortest V-V intervals out of a predetermined numberof V-V intervals, e.g., excluding about 2 of the shortest V-V intervalsout of about 10 intervals.

The A-rate calculation criteria may be modified by excluding one or moreof the longest A-A intervals out of a predetermined number of A-Aintervals, e.g., excluding about 2 of the longest A-A intervals out ofabout 10 intervals. The A-rate and/or V-rate is recalculated using themodified criteria and the relationship between the A-rate and the V-rateis determined. The cardiac rhythm is reclassified 1190 based on themodified A-rate and/or V-rate parameters.

In one implementation, the A=V algorithm is reevaluated to determinebias toward A>V or V>A. After the reevaluation, if A>V, then the FCC maybe lowered 1185 and the morphology classified based on the lowered FCC.

The discrimination procedures illustrated by blocks 1182, 1183, 1184,and 1185 above may be used in combination with examining cross chambertiming shift 1186 of the rate and shock channel electrograms. The timingshift between the ventricular rate channel and the ventricular shockchannel may be evaluated to determine if the shock channel signal hasshifted with respect to the rate channel signal. For example, the timingshift of feature points of the shock channel may be checked, such as bychecking the timing difference between the rate channel fiducial pointand the peak of the shock EGM. If the timing difference is changed morethan about 1 sample point from that of the template, then the shockchannel is time-shifted with respect to the rate channel. For example,the timing shift may be in the range of about 5 to about 15 ms in eitherdirection. The correlation coefficient is calculated using thetime-shifted shock channel.

After a rhythm has been classified, the rhythm may transition from onetype of rhythm to another type of rhythm. For example, a rhythmpreviously classified as SVT may transition from SVT to VT or anothertype of SVT. Identification of rhythm transitions may be desirable toallow the system to deliver an appropriate therapy for rhythms that areresponsive to therapy. Rhythm transitions may be identified using arules-based methodology. In some implementations, it is desirable tostructure the rules so that there is a certain amount of delay orhysteresis in performing the rhythm identification. This hysteresis inthe rhythm identification process helps to avoid oscillations back andforth between two or more rhythm identification decisions. As with theinitial rhythm identification processes described above, rhythmtransitions may be identified using multiple rhythm discriminators. Whenmore than one independent rhythm discriminator is used, one rhythmdiscriminator may detect a rhythm transition, while another rhythmdiscriminator does not detect a change. If the results of the rhythmdiscriminators conflict with regard to rhythm transition, the system mayimplement one or more procedures to resolve the conflict.

In one example of rhythm transition, the rhythm determined by theinterval-based discriminators (V-rate, V>A algorithm, rhythm onset,rhythm stability) is unchanged but the morphology of the rhythm changes.For example, if a morphology template is used, the rhythm which waspreviously correlated to the template becomes uncorrelated. When acorrelated rhythm is suddenly uncorrelated, if the interval-based rhythmdecision is consistent, the rhythm is possibly not changed. However, ifthe rate is suddenly changed and stable or V>A, the rhythm is changed toVT. Another possibility is that an SVT rhythm is changed to a differenttype of SVT.

In one embodiment, the following set of rules may be implemented toidentify rhythm transitions from SVT to VT:

Rule 1: If an SVT rhythm that was previously A=V or A>V and correlatedto the SVR template changes to uncorrelated and becomes or remains A>V,then the processes described in connection with FIG. 8A may be used toreassess the rhythm. This rhythm transition rule is illustrated byblocks 1220-1255 of FIG. 12. An A=V or A>V rhythm that is correlated1210 to the SVR template is classified 1215 as SVT. This rhythm becomes1220 uncorrelated to the SVR template and becomes or remains A>V. Thesystem determines 1225 if a rhythm transition has occurred. As describedin connection with block 882 of FIG. 8A, the template correlationthreshold may be lowered 1230 and the morphology of the rhythm comparedto the SVR template using the lower correlation threshold. If the rhythmis correlated 1235 to the SVR template using the lower correlationthreshold, then no transition has occurred and the rhythm remains 1240SVT.

In one optional implementation, if the rhythm is not correlated 1235 tothe SVR template using the lower correlation threshold, then the systemdetermines that a rhythm transition has occurred 1245 and the rhythm isreclassified as VT.

In another optional implementation, if the rhythm is not correlated 1235to the SVR template using the lower correlation threshold, then additionrhythm identification processes, such as those described in connectionwith blocks 884, 885, 886, and 888 of FIG. 8A may be employed 1250. Therhythm is reassessed 1255 using information acquired from the additionalrhythm identification processes.

Rule 2: If an SVT rhythm that was previously A>V becomes A=V anduncorrelated and is stable, then the processes 982, 986, 988, 1082,1086, 1088 described in connection with FIG. 9 or 10 may be used toidentify the rhythm.

Rule 3: If an SVT rhythm that was previously A=V stays A=V but becomesuncorrelated and is stable, then the rhythm may be evaluated using alower correlation threshold or by examining the cross chamber timingshift of the rate and shock channel electrograms as described herein. Ifthe rhythm is correlated to the SVR template using the lower correlationthreshold or using the timing shift, then the rhythm remains SVT.However, if the rate is accelerated, then a rhythm transition from SVTto VT may have occurred.

Rule 4: If an SVT rhythm that was previously A>V or A=V becomesuncorrelated, and becomes or remains A=V and is unstable, then anycombination of the discrimination processes 1182, 1183, 1184, 1185, 1186outlined in FIG. 11 may be used to determine if a rhythm transition hasoccurred.

The rules discussed above illustrate a rules-based methodology that maybe used to identify rhythm transitions in accordance with embodiments ofthe invention. Another example of a rules based methodology foridentifying rhythm transitions is illustrated by Table 1 and the rulesthat follow.

Table 1 illustrates possible output states of interval-baseddiscriminators for a number of example cases that will be used toillustrate a rules-based methodology for detecting or confirming rhythmtransitions.

TABLE 1 Case 1 2 3 4 5 6 7 8 V > A Algorithm A > V A > V A > V A > V A =V A = V A = V A = V Sudden Onset True True False False True True FalseFalse V rate Stable True False True False True False True False

Case 1 and 3: A>V and a Correlated and Stable Rhythm is SuddenlyUncorrelated.

In each case listed in Table 1, the rhythm is correlated to the SVRtemplate and initially classified as SVT. For example, Case 1 identifiesa rhythm that was initially classified as SVT, with morphologycorrelated to the SVR template, A>V, sudden onset, and stable V-rate.Case 2 identifies a rhythm that was initially classified as SVT, withmorphology correlated to the SVR template, A>V, without sudden onset,and stable V-rate.

For cases 1 and 3, the following rules may be applied to determine if arhythm shift has occurred when A>V and a correlated and stable rhythm issuddenly uncorrelated:

Rule 1: If the AV pattern and V rate are consistent, the type of rhythmis not changed. The possible reasons for non-correlation to the SVRtemplate are morphological change or the cross chamber timing shift ofthe rhythm (rate channel vs. shock channel signal shift) duringsustaining SVT at high heart rate. One or more processes for confirmingthat the rhythm remains unchanged may be implemented. For example, thetiming shift of feature points of the shock channel may be checked, suchas by checking the timing difference between the rate channel fidicialpoint and the peak of the shock EGM. If the timing difference is changedmore than about 5 ms from that of the template, then the shock channelis shifted with respect to the rate channel and the correlationcoefficient is calculated. If the rhythm is correlated with the extendedshift, then the system confirms that it the rhythm is correlated to theSVR template.

Alternatively, a high rate SVR template or a second SVR template can beused and the correlation coefficient calculated. If the rhythm iscorrelated to the high rate SVR template, this indicates that there isno change in the rhythm. Further, confirmation that the rhythm isunchanged may be implemented by easing conditions of correlation such asby lowing the correlation coefficient threshold or the number ofcorrelated beats in a window for SVT.

Rule 2: When the rhythm becomes morphologically uncorrelated to the SVRtemplate, and if the rate is suddenly changed and a previously stablerhythm becomes unstable, then the rhythm is most likely changed from astable SVT to an unstable SVT. The morphological variation or CCT shiftmay be checked to confirm the rhythm change from stable to unstable SVT.

Case 2 and 4: A>V. Correlated and Unstable Rhythm is SuddenlyUncorrelated.

Rule 1: If the AV pattern and V rate are unchanged, the type of rhythmis most likely not changed. One or more processes for confirming thatthe rhythm remains unchanged may be implemented by calculating thecorrelation coefficient using a different template or using extendedshift as described above.

Case 5 and 7: A=V. Correlated and Stable Rhythm is SuddenlyUncorrelated.

Rule 1: If the AV pattern and V rate are unchanged, the type of rhythmis not changed. Confirmation may be implemented by calculating thecorrelation coefficient using a different template or using extendedshift as described above.

Rule 2: When the rhythm becomes suddenly uncorrelated, and if the ratesuddenly changes and becomes unstable, then the rhythm is changed from astable SVT to an unstable SVT or VT. Confirmation of the rhythm changemay be implemented by calculating the correlation coefficient using adifferent template or using extended shift as described above.

Case 6 and 8: A=V, Correlated and Unstable Rhythm is SuddenlyUncorrelated.

Rule 1: If the AV pattern and V rate are unchanged, the type of rhythmis most likely not changed. Confirmation may be implemented bycalculating the correlation coefficient using a different template orusing extended shift as described above.

Rule 2: If the rhythm becomes uncorrelated and if the V rate wassuddenly changed and is unstable, and A>V, then the rhythm hastransitioned to AF. Confirmation may be implemented by calculating thecorrelation coefficient using a different template or using extendedshift as described above.

In another example of rhythm transition, the rhythm determined by amorphology-based discriminator is unchanged but the classificationdetermined by an interval-based discriminator changes. A rules-basedmethodology may be implemented for detecting or confirming rhythmtransitions when the rhythm classification of the morphology-baseddiscriminator is unchanged, but the rhythm classification of theinterval-based discriminator changes from SVT to VT. Rules 1 and 2illustrated below provide yet another example of a rules-basedmethodology for determining rhythm transitions in accordance withembodiments of the invention.

Rule 1: If A>V or A=V is changed to V>A while the rhythm is correlatedto the SVR template, the transition from SVT to VT may be confirmed bythe stability of V>A or another independent algorithm.

Rule 2: If A>V or A=V is unchanged while the V rate or the stability issuddenly changed, the rhythm has possibly changed from one type of SVTto another type of SVT.

It will, of course, be understood that various modifications andadditions can be made to the preferred embodiments discussed hereinabovewithout departing from the scope of the present invention. Accordingly,the scope of the present invention should not be limited by theparticular embodiments described above, but should be defined only bythe claims set forth below and equivalents thereof.

1. A method to identify a tachyarrhythmia rhythm transition, the methodcomprising: monitoring a supraventricular tachyarrhythmia (SVT) rhythmfrom a first point in time to a second point in time, the SVT rhythmhaving at the first point in time, a first atrial-ventricular (A-V)relationship, a first indication of stability, and a first indication ofcorrelation with a morphological template, and the SVT rhythm having atthe second point in time, a second A-V relationship, a second indicationof stability, and a second indication of correlation with themorphological template; identifying whether there is a change in the A-Vrelationship from the first point in time to the second point in time,based on the first A-V relationship and the second A-V relationship;identifying whether there is a change in the indication of stabilityfrom the first point in time to the second point in time, based on thefirst indication of stability and the second indication of stability;identifying whether there is a change in the indication of correlationfrom the first point in time to the second point in time, based on thefirst indication of correlation and the second indication ofcorrelation; and identifying that a tachyarrhythmia rhythm transitionfrom SVT is likely to exist based on whether there are changes in theA-V relationship, the indication of stability, or the indication ofcorrelation from the first point in time to the second point in time. 2.The method of claim 1, wherein the first A-V relationship is A=V or A>V,and the first indication of correlation indicates that the SVT rhythm iscorrelated to a supraventricular rhythm (SVR) template, and wherein thesecond A-V relationship is A>V, and the second indication of correlationindicates that the SVT rhythm is uncorrelated to the SVR template, andwherein identifying that the tachyarrhythmia rhythm transition from SVTis likely to exist comprises: reducing a template correlation threshold;comparing the SVT rhythm with the SVR template using the templatecorrelation threshold; and when the SVT rhythm is not correlated withthe SVR template, identifying that a tachyarrhythmia rhythm transitionfrom SVT to ventricular tachyarrhythmia (VT) is likely to exist.
 3. Themethod of claim 1, wherein the first A-V relationship is A=V or A>V, andthe first indication of correlation indicates that the SVT rhythm iscorrelated to a supraventricular rhythm (SVR) template, and wherein thesecond A-V relationship is A>V, and the second indication of correlationindicates that the SVT rhythm is uncorrelated to the SVR template, andwherein identifying that the tachyarrhythmia rhythm transition from SVTis likely to exist comprises: reducing a template correlation threshold;comparing the SVT rhythm with the SVR template using the templatecorrelation threshold; and when the SVT rhythm is not correlated withthe SVR template, identifying that a tachyarrhythmia rhythm transitionfrom SVT to ventricular tachyarrhythmia (VT) is likely to exist.
 4. Themethod of claim 3, comprising using a rhythm identification process toconfirm the transition to VT.
 5. The method of claim 1, wherein thefirst A-V relationship is A=V or A>V, and the first indication ofcorrelation indicates that the SVT rhythm is correlated to a SVRtemplate, and the first indication of stability indicates that the SVTrhythm is unstable, and wherein the second A-V relationship is A=V, andthe second indication of correlation indicates that the SVT rhythm isuncorrelated to the SVR template, and wherein identifying that thetachyarrhythmia rhythm transition from SVT is likely to exist comprises:identifying that a rhythm transition has most likely not changed; andconfirming a status of the rhythm transition by calculating acorrelation coefficient using a different template than the SVRtemplate.
 6. The method of claim 1, wherein the first A-V relationshipis A=V or A>V, and the first indication of correlation indicates thatthe SVT rhythm is correlated to a SVR template, and the first indicationof stability indicates that the SVT rhythm is unstable, and wherein thesecond A-V relationship is A=V, and the second indication of correlationindicates that the SVT rhythm is uncorrelated to the SVR template, andwherein identifying that the tachyarrhythmia rhythm transition from SVTis likely to exist comprises: identifying that a rhythm transition hasmost likely not changed; and confirming a status of the rhythmtransition by calculating a correlation coefficient using an extendedshift of feature points in a shock channel.
 7. The method of claim 1,wherein the first A-V relationship is A=V or A>V, and the firstindication of correlation indicates that the SVT rhythm is correlated toa SVR template, and the first indication of stability indicates that theSVT rhythm is unstable, and wherein the second A-V relationship is A>V,and the second indication of correlation indicates that the SVT rhythmis uncorrelated to the SVR template, and wherein identifying that thetachyarrhythmia rhythm transition from SVT is likely to exist comprises:identifying that a rhythm transition has most likely transitioned toatrial fibrillation (AF); and confirming a status of the rhythmtransition by calculating a correlation coefficient using at least oneof a different template than the SVR template or an extended shift offeature points in a shock channel.
 8. A non-transitory machine-readablemedium comprising instructions to identify a tachyarrhythmia rhythmtransition, which when executed by a machine, cause the machine to:monitor a supraventricular tachyarrhythmia (SVT) rhythm from a firstpoint in time to a second point in time, the SVT rhythm having at thefirst point in time, a first atrial-ventricular (A-V) relationship, afirst indication of stability, and a first indication of correlationwith a morphological template, and the SVT rhythm having at the secondpoint in time, a second A-V relationship, a second indication ofstability, and a second indication of correlation with the morphologicaltemplate; identify whether there is a change in the A-V relationshipfrom the first point in time to the second point in time, based on thefirst A-V relationship and the second A-V relationship; identify whetherthere is a change in the indication of stability from the first point intime to the second point in time, based on the first indication ofstability and the second indication of stability; identify whether thereis a change in the indication of correlation from the first point intime to the second point in time, based on the first indication ofcorrelation and the second indication of correlation; and identify thata tachyarrhythmia rhythm transition from SVT is likely to exist based onwhether there are changes in the A-V relationship, the indication ofstability, or the indication of correlation from the first point in timeto the second point in time.
 9. The non-transitory machine-readablemedium of claim 8, wherein the first A-V relationship is A=V or A>V, andthe first indication of correlation indicates that the SVT rhythm iscorrelated to a supraventricular rhythm (SVR) template, and wherein thesecond A-V relationship is A>V, and the second indication of correlationindicates that the SVT rhythm is uncorrelated to the SVR template, andwherein the instructions to identify that the tachyarrhythmia rhythmtransition from SVT is likely to exist comprise instructions to: reducea template correlation threshold; compare the SVT rhythm with the SVRtemplate using the template correlation threshold; and when the SVTrhythm is not correlated with the SVR template, identify that atachyarrhythmia rhythm transition from SVT to ventriculartachyarrhythmia (VT) is likely to exist.
 10. The non-transitorymachine-readable medium of claim 8, wherein the first A-V relationshipis A=V or A>V, and the first indication of correlation indicates thatthe SVT rhythm is correlated to a supraventricular rhythm (SVR)template, and wherein the second A-V relationship is A>V, and the secondindication of correlation indicates that the SVT rhythm is uncorrelatedto the SVR template, and wherein the instructions to identify that thetachyarrhythmia rhythm transition from SVT is likely to exist compriseinstructions to: reduce a template correlation threshold; compare theSVT rhythm with the SVR template using the template correlationthreshold; and when the SVT rhythm is not correlated with the SVRtemplate, identify that a tachyarrhythmia rhythm transition from SVT toventricular tachyarrhythmia (VT) is likely to exist.
 11. Thenon-transitory machine-readable medium of claim 10, comprisinginstructions to use a rhythm identification process to confirm thetransition to VT.
 12. The non-transitory machine-readable medium ofclaim 8, wherein the first A-V relationship is A=V or A>V, and the firstindication of correlation indicates that the SVT rhythm is correlated toa SVR template, and the first indication of stability indicates that theSVT rhythm is unstable, and wherein the second A-V relationship is A=V,and the second indication of correlation indicates that the SVT rhythmis uncorrelated to the SVR template, and wherein the instructions toidentify that the tachyarrhythmia rhythm transition from SVT is likelyto exist comprise instructions to: identify that a rhythm transition hasmost likely not changed; and confirm a status of the rhythm transitionby calculating a correlation coefficient using a different template thanthe SVR template.
 13. The non-transitory machine-readable medium ofclaim 8, wherein the first A-V relationship is A=V or A>V, and the firstindication of correlation indicates that the SVT rhythm is correlated toa SVR template, and the first indication of stability indicates that theSVT rhythm is unstable, and wherein the second A-V relationship is A=V,and the second indication of correlation indicates that the SVT rhythmis uncorrelated to the SVR template, and wherein the instructions toidentify that the tachyarrhythmia rhythm transition from SVT is likelyto exist comprise instructions to: identify that a rhythm transition hasmost likely not changed; and confirm a status of the rhythm transitionby calculating a correlation coefficient using an extended shift offeature points in a shock channel.
 14. The non-transitorymachine-readable medium of claim 8, wherein the first A-V relationshipis A=V or A>V, and the first indication of correlation indicates thatthe SVT rhythm is correlated to a SVR template, and the first indicationof stability indicates that the SVT rhythm is unstable, and wherein thesecond A-V relationship is A>V, and the second indication of correlationindicates that the SVT rhythm is uncorrelated to the SVR template, andwherein the instructions to identify that the tachyarrhythmia rhythmtransition from SVT is likely to exist comprise instructions to:identify that a rhythm transition has most likely transitioned to atrialfibrillation (AF); and confirm a status of the rhythm transition bycalculating a correlation coefficient using at least one of a differenttemplate than the SVR template or an extended shift of feature points ina shock channel.
 15. A tachyarrhythmia detection and classificationsystem, comprising: a sensing circuit, adapted to sense at least onecardiac signal; and an implant controller, coupled to the sensingcircuit, the implant controller including: an arrhythmia discriminatormodule, the arrhythmia discriminator module adapted to: monitor asupraventricular tachyarrhythmia (SVT) rhythm from a first point in timeto a second point in time, the SVT rhythm having at the first point intime, a first atrial-ventricular (A-V) relationship, a first indicationof stability, and a first indication of correlation with a morphologicaltemplate, and the SVT rhythm having at the second point in time, asecond A-V relationship, a second indication of stability, and a secondindication of correlation with the morphological template; identifywhether there is a change in the A-V relationship from the first pointin time to the second point in time, based on the first A-V relationshipand the second A-V relationship; identify whether there is a change inthe indication of stability from the first point in time to the secondpoint in time, based on the first indication of stability and the secondindication of stability; identify whether there is a change in theindication of correlation from the first point in time to the secondpoint in time, based on the first indication of correlation and thesecond indication of correlation; and identify that a tachyarrhythmiarhythm transition from SVT is likely to exist based on whether there arechanges in the A-V relationship, the indication of stability, or theindication of correlation from the first point in time to the secondpoint in time.
 16. The system of claim 15, wherein the first A-Vrelationship is A=V or A>V, and the first indication of correlationindicates that the SVT rhythm is correlated to a supraventricular rhythm(SVR) template, and wherein the second A-V relationship is A>V, and thesecond indication of correlation indicates that the SVT rhythm isuncorrelated to the SVR template, and wherein the arrhythmiadiscriminator module is adapted to identify that the tachyarrhythmiarhythm transition from SVT is likely to exist by: reducing a templatecorrelation threshold; comparing the SVT rhythm with the SVR templateusing the template correlation threshold; and when the SVT rhythm is notcorrelated with the SVR template, identifying that a tachyarrhythmiarhythm transition from SVT to ventricular tachyarrhythmia (VT) is likelyto exist.
 17. The system of claim 15, wherein the first A-V relationshipis A=V or A>V, and the first indication of correlation indicates thatthe SVT rhythm is correlated to a supraventricular rhythm (SVR)template, and wherein the second A-V relationship is A>V, and the secondindication of correlation indicates that the SVT rhythm is uncorrelatedto the SVR template, and wherein the arrhythmia discriminator module isadapted to identify that the tachyarrhythmia rhythm transition from SVTis likely to exist by: reducing a template correlation threshold;comparing the SVT rhythm with the SVR template using the templatecorrelation threshold; and when the SVT rhythm is not correlated withthe SVR template, identifying that a tachyarrhythmia rhythm transitionfrom SVT to ventricular tachyarrhythmia (VT) is likely to exist.
 18. Thesystem of claim 17, wherein the arrhythmia discriminator module isadapted to use a rhythm identification process to confirm the transitionto VT.
 19. The system of claim 15, wherein the first A-V relationship isA=V or A>V, and the first indication of correlation indicates that theSVT rhythm is correlated to a SVR template, and the first indication ofstability indicates that the SVT rhythm is unstable, and wherein thesecond A-V relationship is A=V, and the second indication of correlationindicates that the SVT rhythm is uncorrelated to the SVR template, andwherein the arrhythmia discriminator module is adapted to identify thatthe tachyarrhythmia rhythm transition from SVT is likely to exist by:identifying that a rhythm transition has most likely not changed; andconfirming a status of the rhythm transition by calculating acorrelation coefficient using a different template than the SVRtemplate.
 20. The system of claim 15, wherein the first A-V relationshipis A=V or A>V, and the first indication of correlation indicates thatthe SVT rhythm is correlated to a SVR template, and the first indicationof stability indicates that the SVT rhythm is unstable, and wherein thesecond A-V relationship is A=V, and the second indication of correlationindicates that the SVT rhythm is uncorrelated to the SVR template, andwherein the arrhythmia discriminator module is adapted to identify thatthe tachyarrhythmia rhythm transition from SVT is likely to exist by:identifying that a rhythm transition has most likely not changed; andconfirming a status of the rhythm transition by calculating acorrelation coefficient using an extended shift of feature points in ashock channel.
 21. The system of claim 15, wherein the first A-Vrelationship is A=V or A>V, and the first indication of correlationindicates that the SVT rhythm is correlated to a SVR template, and thefirst indication of stability indicates that the SVT rhythm is unstable,and wherein the second A-V relationship is A>V, and the secondindication of correlation indicates that the SVT rhythm is uncorrelatedto the SVR template, and wherein the arrhythmia discriminator module isadapted to identify that the tachyarrhythmia rhythm transition from SVTis likely to exist by: identifying that a rhythm transition has mostlikely transitioned to atrial fibrillation (AF); and confirming a statusof the rhythm transition by calculating a correlation coefficient usingat least one of a different template than the SVR template or anextended shift of feature points in a shock channel.